My Wife tried to do an in app purchase but instead of her gmail showing up her twin's did instead and needed her password as well how can it be switched so only her email and password can be used ?
There should be a page in the settings app called Accounts. All the accounts stored on the device will be listed there. To remove an account, tap on the account name and then tap the remove account button
See this link for more info Add or remove an account on Android